The Pennsylvania State University,
Keywords: Kehao Zhang, Nicholas J. Borys, Ganesh R. Bhimanapati, Baoming Wang, Teague A. Williams , Ke Wang, Aman Haque , Edward S. Barnard, P. James Schuck, Joshua A. Robinson
Summary:The field of two-dimensional materials is currently quite vogue, with the potential to revolutionize photovoltaics, photodetectors, and transistor. However, the predicted outstanding properties are limited by defects and doping, with many attempts at ion implementation, annealing, chemical doping and super acid treatments to optimize the properties. All of the developments to date, however, have missed a critical aspect in their research: the substrate. Sapphire, a well-known substrate for semiconductor synthesis, now plays an important role in the growth of 2D materials due to its high crystallinity and tunable surface chemistry. However, the impact of the chemical and structural characteristics of the sapphire surface on the structural and optical properties of the powder vaporization (PV)-grown MoS2 has not been fully explored. In this work, we investigate the impact of the substrate surface energy and chemistry on the crystalline orientation and the optical properties of synthetic monolayer MoS2. Thermal treatments are used to systematically tune the surface chemistry (and thus energy) of the sapphire, where higher sapphire surface energy facilitates the alignment of MoS2 monolayer domains to the underlying sapphire structure. As aligned domains merge, grain boundaries remain optically isotropic and indistinguishable from the domain center. In contrast, misaligned MoS2 domain boundaries exhibit increased density of defects, with suppressed non-radiative relaxation processes that lead to enhanced photoluminescence and prolonged excited-state lifetimes. Finally, we demonstrate that enhanced photoluminescence and carrier lifetime can be engineered by tuning the surface termination of the sapphire substrate, where an oxygen terminated sapphire surface results in >100 enhancement of brightness compared to that of monolayer MoS2 grown on an aluminum terminated surface due to oxygen doping. The results presented here offer deeper insight into the how careful preparation of the substrate prior to 2D layer growth can substantially enhance the structural and photophysical properties of PV-grown monolayer MoS2.